An electric power steering apparatus that assist-controls a steering system of a vehicle by using a rotational torque of a motor, applies a driving force of the motor as a steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. And then, in order to supply a current to the motor so that the motor generates a desired torque, an inverter comprised of an FET bridge is used in a motor drive circuit.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (an ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and controls a current I supplied to the motor 20 based on a voltage command value E obtained by performing compensation and so on with respect to the current command value in a current control section. Furthermore, it is also possible to receive the velocity V from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the velocity V detected by the velocity sensor 12 are inputted into a current command value calculating section 101. The current command value calculating section 101 decides a current command value Iref1 that is the desired value of the current I supplied to the motor 20 based on the steering torque T and the velocity V and by means of an assist map and so on. The current command value Iref1 is added in an addition section 102A and then the added value is inputted into a current limiting section 103 as a current command value Iref2. A current command value Iref3 that is limited the maximum current, is inputted into a subtraction section 102B, and a deviation Is (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back, is calculated. The deviation Is is inputted into a current control section 104 that performs PI control and so on. The voltage command value E that characteristic improvement is performed in the current control section 104, is inputted into a PWM control section 105. Furthermore, the motor 20 is PWM-driven through an inverter 106 serving as a drive section. The current value Im of the motor 20 is detected by a current detector 106A within the inverter 106 and is fed back to the subtraction section 102B. In general, the inverter 106 uses EFTs as switching elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation section 110 is added in the addition section 102A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation section 110 adds a self-aligning torque (SAT) 113 and an inertia 112 in an addition section 114, further adds the result of addition performed in the addition section 114 and a convergence 111 in an addition section 115, and then outputs the result of addition performed in the addition section 115 as the compensation signal CM.
In the case that the motor 20 is a 3-phase (U-phase, V-phase and W-phase) brushless motor, details of the PWM control section 105 and the inverter 106 are a configuration such as shown in FIG. 3. The electric power is supplied to the PWM control section 105 and the inverter 106 from the battery 13 through a power switch 14. The PWM control section 105 comprises a duty calculating section 105A that calculates PWM duty command values D1˜D6 of three phases according to a given expression based on the voltage command value E, an upper-FET gate driving section 105B1 that switches ON/OFF after driving each gate of upper-FET1˜upper-FET3 by the PWM duty command values D1˜D3, a lower-FET gate driving section 105B2 that switches ON/OFF after driving each gate of lower-FET4˜lower-FET6 by the PWM duty command values D4˜D6, and a booster power circuit 105C for boosting a power-supply voltage of the upper-FET gate driving section 105B1. The upper-FET gate driving section 105B1 and the lower-FET gate driving section 105B2 comprise a U-phase gate driving section, a V-phase gate driving section, and a W-phase gate driving section, respectively. Further, the inverter 106 comprises a three-phase bridge having top and bottom arms comprised of upper-FET1 and lower-FET4 of U-phase, top and bottom arms comprised of upper-FET2 and lower-FET5 of V-phase, and top and bottom arms comprised of upper-FET3 and lower-FET6 of W-phase, and drives the motor 20 by FET1˜FET6 being switched ON/OFF based on the PWM duty command values D1˜D6.
In such an electric power steering apparatus, as the booster power circuit (105C), for example, Japanese Published Unexamined Patent Application No.2004-173336 A (Patent Document 1) generates a booster power by using a charge pump circuit, and as another example, Japanese Published Unexamined Patent Application No.2005-51926 A (Patent Document 2) generates the booster power by using a bootstrap circuit.